Polyisocyanates containing biuret groups are known and are used as starting materials for high-grade, light-stable lacquers. They may be obtained, for example, from diisocyanates and water (German Auslegeschrift No. 1,101,394 or U.S. Pat. No. 3,124,605), hydrogen sulphide (German Auslegeschrift No. 1,165,580), formic acid (German Auslegeschrift No. 1,174,760), tertiary alcohols (German Auslegeschrift Nos. 1,543,178 and 1,931,055 or U.S. Pat. No. 3,358,010) or monoamines (German Offenlegungsschrift No. 2,308,015 and U.S. Pat. No. 3,903,127).
In this conventional process, amino groups are initially formed from some of the isocyanate groups and then further react with excess diisocyanate, via the corresponding urea diisocyanates, to form biuret polyisocyanates. The conversion of the isocyanate groups into amino groups is always accompanied by the formation of gaseous secondary products, such as carbon dioxide, carbon monoxide, carbon sulphoxide or olefins, whose elimination may give rise to emission problems. In the heterogeneous reaction of diisocyanates with water, an additional problem lies in the formation of insoluble polyureas which are difficult to separate off. However, a particular disadvantage of these known processes is that some of the isocyanate groups in the diisocyanates, used as starting material, are initially destroyed through the formation of amino groups.
Accordingly, there has also been no shortage of attempts to produce polyisocyanates containing biuret groups by directly reacting diamines with diisocyanates without any elimination of volatile secondary products and without the destruction of isocyanate groups for amine formation.
Due to the high reactivity of aliphatic amino groups to isocyanate groups, considerable practical difficulties were encountered. The most difficult is the reaction of primary diamines with diisocyanates because of the great tendency for insoluble polyureas and cross-linked products to be formed.
As may be seen from German Offenlegungsschrift No. 2,261,065, for example, uneconomically long afterheating at elevated temperature is required for completing the reaction where readily available starting materials, such as hexamethylene diamine and hexamethylene diisocyanate, are used. This greatly impairs the properties of the end products, particularly their natural color. According to German Offenlegungsschrift No. 2,609,995 and U.S. Pat. No. 4,147,714, these disadvantages may be obviated by introducing the diamine into the diisocyanate in vapor form under carefully controlled conditions. In this process, however, steps have to be taken to ensure that no diisocyanate enters the feed pipe because otherwise blockages attributable to urea formation may rapidly occur.
In these conventional processes, formation of the biuret polyisocyanates is accompanied by rearrangement reactions by which the diamine used is converted into the corresponding diisocyanate depending upon the NCO/NH.sub.2 -ratio. As a result, mixtures of different diisocyanates accumulate as distillates during separation of the unreacted diisocyanate from the biuret polyisocyanate unless diamines and diisocyanates of the same constitution are used. Another disadvantage is that more or less large quantities of the diisocyanate formed from the diamine by trans-biuretization remain behind as a monomer component in the biuret polyisocyanate itself.
According to German Offenlegungsschrift No. 2,010,887 and U.S. Pat. No. 3,862,973, the direct reaction of secondary diamines may be carried out with diisocyanates. By this process, it is possible to produce primarily biuret polyisocyanates containing aromatically bound isocyanate groups which, unfortunately, are not suitable for high-quality light-stable lacquers. Where aliphatic diamines are used in combination with aliphatic diisocyanates, the corresponding bis-urea diisocyanates are spontaneously formed in this known process, but unfortunately cannot be readily reacted with more diisocyanate to form higher biuret polyisocyanates.
An object of the present invention is to provide a new process by which it is readily possible to produce high-quality, modified aliphatic polyisocyanates which combine the advantages of known biuret polyisocyanates without the process being attended by the above-mentioned disadvantages of the conventional processes.
This object is achieved by the process according to the present invention as described in the following.